Solves this e-mail I received from Google Webmaster Tools today:

Google has detected that some pages on your site have implemented the rel-alternate-hreflang tag incorrectly. In particular, there seems to be a problem with incorrect language and region codes, or incorrect bi-directional linking (if page A links with hreflang to page B, there must be a link back from B to A as well). Google uses the hreflang attributes to serve the correct language or regional URL to the right users in search results.

Until you correct the errors on the hreflang links, your website will not benefit from additional language or region targeting.

Fix this problem now:

Find incorrect hreflang implementation
Use the examples provided in the International Targeting feature in Search Console to get a sample of pages with incorrect hreflang implementation.